The combination of Importance and Sensitivity Analysis (ISA) with Fault Tree Analysis (FTA) constitutes a very powerful tool to improve the design of critical systems. Current ISA methodologies analyse different top-events sequentially to identify, through the components importance measures, the weakest parts of the system which require further design considerations. The present paper describes a new method for on-line Importance and Sensitivity Analysis which allows identifying design solutions that satisfy predefined failure probability goals for all Top-events. The originality of this method relies on the possibility to rapidly obtain a uniformly protected system, i.e. without weak points as well as without "over-reliable" functions.